This invention relates to fabrication of semiconductor devices using low temperature plasma, and more particularly to a plasma processing apparatus suited for carrying out the respective techniques of CVD, etching, sputtering, ashing, etc. at high speed.
Devices using low-temperature plasma roughly includes two categories consisting of one of applying a high frequency voltage at about 10 KHz to 30 MHz to one of parallel plate electrodes in vacuum to create plasma (see "HANDOTAI KENKYU" No. 18, pages 121-137, 145-169); and the other of introducing a microwave at 2.45 GHz into a vacuum chamber to create plasma. Conventionally, the device belonging to the former category has been mainly used.
On the other hand, the miniaturization of the semiconductor devices has caused a problem that the bombardment of ions which is generated in plasmaprocessing thereof disadvantageously influences the device performance. It has been also required to increase the processing speed in order to improve the processing capability.
In order to increase the processing speed, only increasing the density of plasma or the radical concentration thereof, which means the concentration of active paticles immediately before the ionization, is not sufficient. Specifically, the energy of ions plays an important role in dry etching using plasma and plasma CVD. In the case of dry etching, if the ion energy is too high, the underlying film will be cut or the crystal structure will be badly influenced, thus deteriorating the device performance. On the other hand, if the ion energy is too low, the polymer formed on the etched face will not be sufficiently removed, thus reducing the etching speed; or inversely a passivation film of the polymer will not be formed but the side of the pattern will be etched, thus reducing the dimension accuracy of the pattern.
Also in the plasma CVD, the magnitude of ion energy influences the quality of the formed film in such a way that low ion energy leads to coarse film composition while high ion energy leads to dense film composition.
Thus, it is indispensable for the future plasma processing to densify the plasma and also properly control the ion energy. Examples of such a plasma processing are the systems of producing plasma using microwaves such as disclosed in JP-A-56-13480 (U.S. Pat. No. 4,492,620), and JP-A-56-96841.
When plasma is to be produced using microwaves, only the emitting of the microwaves, which is generated by means of a magnetron, into a highly evacuated plasma production chamber cannot satisfactorily produce plasma. This is because the field strength of the microwaves generated is not so high that the electrons are not supplied with sufficient energy. Thus, in order to sufficiently produce the plasma using the microwaves, there have been proposed two techniques; one is to supply the electrons with energy in a cyclotron resonance condition occurring when the cyclotron frequency, which is the orbitral motion frequency when electrons travel in the plane perpendicular to the magnetic field, and the frequency of the microwaves are made equal to each other, and the other is to supply the electrons with the energy with the electric field strength increased by emitting the microwaves into a cavity resonator to increase the amplitude of the microwaves. The former, which is generally referred to as an ECR (Electron Cyclotron Resonance) technique, is disclosed in the above JP-A-56-13480 and U.S. Pat. No. 4,492,620. The latter is disclosed in the above JP-A-56-96841.
In the case of the these techniques, the plasma generated through the microwaves includes the electrons directly supplied with energy from the microwaves so that the voltage across the sheath formed between the plasma and the substrate does not almost vary. Thus, by appropriating controlling the voltage across the sheath with the high frequency voltage being applied to the electrode on which a substrate is placed, it is possible to provide the plasma with the higher density and the ion energy properly controlled for the high speed processing.
Again, in the plasma processing the ion energy plays an important role. However, in the prior art ECR technique disclosed in the above JP-A-56-13480, when a high frequency voltage is applied to the electrode on which the substrate is placed, the high frequency current flows towards the surrounding processing chamber since the ground electrode is not arranged on the side opposite to the above electrode. Thus, the effect of ion energy on the substrate is greater on its periphery and is smaller on its center. Therefore, the entire substrate cannot be processed under a uniform condition.
In the prior art technique of using a cavity resonator such as disclosed in the above JP-A-56-96841, the plasma is produced in a cavity resonator so that when the plasma is produced, the wavelength of the microwaves is changed according to the plasma density. Thus, the resonance condition is not satisfied and accordingly the plasma will be unstable. More specifically, since the resonance condition is satisfied before the plasma is produced, the electric field strength of the microwaves becomes greater, thus eventually producing the plasma. However, when the plasma density of the plasma thus produced becomes higher, the wavelength of the microwave will be changed. Thus, the resonance condition will not be satisfied, thereby decreasing the electric field strength. Then, the electrons will be supplied with lower energy and so the plasma density will be decreased. Once the plasma density is decreased, the resonance condition will be satisfied again and the plasma density will be higher. Due to such a phenomenon, it is difficult to produce stabilized plasma using the cavity resonator.
Moreover, when the electrode for applying a high frequency voltage is arranged in the cavity resonator in order to control the energy of the ions incident to the substrate from the plasmas, the reflection of the microwave or the like will occur so that the plasma formed will be further unstable.